Cost–benefit analysis of mollusc eating in a shorebird : II. Optimizing gizzard size in the face of seasonal demands

Aiming to interpret functionally the large variationin gizzard masses of red knots Calidris canutus, weexperimentally studied how the digestive processing rate isinfluenced by the size of the gizzard. During their nonbreedingseason, red knots feed on hard-shelled molluscs,which they ingest whole and crush in their gizzard. Inthree experiments with captive birds we tested predictionsof the hypothesis that gizzard size, via the rate of shellcrushing and processing, constrains intake rate in redknots (against the alternative idea that external handlingtimes constrain intake rate). Gizzard size withinindividual birds was manipulated by varying the hardnessof the diet on offer, and was confirmed byultrasonography. The results upheld the ‘shell-crushinghypothesis’ and rejected the ‘handling time hypothesis’.Intake rates on with-shell prey increased with gizzard size,and decreased with shell mass per prey. Intake rates onsoft (without shell) prey were higher than on with-shellprey and were unaffected by gizzard size. Offering preythat were heavily shelled relative to their flesh mass led toenergy intake rates that were marginally sufficient tobalance the daily energy budget within the time that isnaturally available in a tidal system. We predicted theoptimal gizzard sizes that are required to either (1)balance energy income with energy expenditure, or (2) tomaximise net daily energy intake. The gizzard mass offree-living red knots in the Wadden Sea is such that itmaximises daily net energy intake in spring when fuellingfor migration, while it balances energy budget throughoutthe remainder of the year.